The present invention concerns a capacitive device for adjusting the frequency of a quartz crystal oscillator.
A quartz crystal oscillator usually comprises a trimmer, that is to say, a variable reactive impedance device, mounted in series or in parallel with the quartz crystal for correcting the frequency variations due to the manufacturing tolerances of the quartz crystal and the oscillator drive circuit, and also due to unstable features such as ageing, shocks and the like.
An oscillator is known in which the trimmer is constituted by one of the electrodes of the quartz crystal and a deformable conductive surface located opposite thereto, thus fulfilling the function of counterelectrode to earth. By varying the distance between the electrode and the deformable surface, the value of the capacitor connected in parallel with the quartz is modified, thus making it possible to adjust the frequency of the oscillator in a relatively restricted range.
Whilst reducing the parasitic capacitance associated with a conventional trimmer, this embodiment has several disadvantages: too small adjusting range; a non-linear relationship between the position of the deformable surface and the value of the capacitor; mechanical constraints on the connection between deformable and ceramic surface, large volume occupied by the adjusting screw.